1. Field of the Invention
The present invention generally relates to circuits for starting asynchronous motors powered by an A.C. voltage and, more specifically, to a circuit for controlling a triac driving an auxiliary winding of an asynchronous motor for starting thereof.
An example of application of the present invention relates to compressors which generally comprise such asynchronous motors powered by the A.C. mains voltage.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional example of a circuit for controlling windings of an asynchronous motor. For simplification, the motor has been symbolized by a main winding Lm and an auxiliary winding Ls used for the starting. Main winding Lm is intended to be powered by an A.C. voltage Vac applied between two terminals 1 and 2. A switch K (for example, controlled by a thermostat Th, by the user, etc.) is interposed in series with winding Lm between terminals 1 and 2.
To start an asynchronous motor, it is necessary to create torque by means of a phase shift or by injecting a current greater than the current absorbed by winding Lm. Such is the function of auxiliary winding Ls, connected in parallel with the main winding.
This auxiliary or starting winding Ls is not intended to operate continuously. This is why it is generally associated with a resistive element 3 of positive temperature coefficient (PTC) having its resistance increasing along with temperature. Element 3 enables disconnecting the auxiliary winding once the motor has started, the current then flowing in the auxiliary winding being sufficient for the resistance of element 3 to be considered as opening the branch of the auxiliary winding.
To avoid that element 3 continuously dissipate power in the circuit, it is generally series-connected with a triac T. Thus, as illustrated in FIG. 1, auxiliary winding Ls in series with element 3 and triac T are connected in parallel with main winding Lm. The gate of triac T is connected to the junction point of a resistor R and a capacitor C, connected between terminals 1 and 2, a rectifying diode D being interposed between terminal 1 and resistor R with its anode on the side of terminal 1.
The circuit of FIG. 1 is described in document EP-A-0571956.
When a voltage Vac is applied between terminals 1 and 2 and switch K is turned on, the current which flows in winding Ls helps provide a torque to the motor to start it. In parallel, triac T is turned on by the gate current provided thereto by diode D and resistor R. A circuit 6 is used to discharge capacitor C to turn off triac T after a given time, which disconnects winding Ls. This time set by circuit 6 corresponds to the starting time. The starting time (conduction of winding Ls) is set by the time constant brought by resistor R and capacitor C. In such a circuit, resistive element 3 of positive temperature coefficient is used as a security for the case where triac T would be defective.
A disadvantage of the circuit of FIG. 1 is that it is used only once, on powering-on of the assembly. Due to the direct connection of diode D to terminal 1, it is no longer used when the thermostat turns off the motor, capacitor C remaining charged.
Even if the starting circuit (diode D, resistor R, capacitor C) were connected downstream of switch K (anode of diode D connected between switch K and windings Lm and Ls), the absence of the discharge circuit of capacitor C would adversely affect the restarting of the motor, after a turning-off/turning-on of the thermostat.
Further, most often, a normally-on switch (not shown in FIG. 1) used as a thermal protection (known under name KLIXON) is interposed between point 4 of interconnection of windings Ls and Lm and switch K. This protection switch is generally internal to the motor so that point 4 is, in practice, not accessible. In such a case, the circuit of FIG. 1 does not enable automatically restarting the motor on turning-on of switch KLIXON, after having undergone a thermal protection opening.
Assemblies (for example, from document U.S. Pat. No. 5,989,289) are also known in which a second resistive element with a positive temperature coefficient is provided to supply the triac gate. A disadvantage of this assembly is that the priming is late in the case where the second resistive element heats up, which generates electromagnetic noise. Another disadvantage of this type of assembly is a halfwave conduction.